Acrylic filaments which are particularly suited for thermal conversion to carbon filaments

ABSTRACT

Improved acrylic filaments are provided having an internal structure capable of yielding improved carbon filaments upon thermal conversion. The acrylilc filaments of the present invention are formed by a specifically defined wet spinning process which produces filaments having a highly fibrillar and dense internal structure. Upon thermal stabilization and subsequent thermal treatment at 1,000* to 2,000*C. a highly developed microporous structure is inherently imparted to the resulting carbon filaments wherein a large number of elongated micropores are disposed between a highly fibrillar internal structure. The resulting carbon filaments have an internal structure capable of increasing the amount of work required to break the same via crack diversion during fracture. Satisfactory strength properties are exhibited even if the internal structure of the improved carbon filaments is accompanied by the presence of gross inhomogenities and structural flaws, such as commonly encountered in carbon filaments of the prior art.

United States Patent 91 Ram et al.-

1 Nov. 5, 1974 ACRYLIC FILAMENTS WHICH ARE PARTICULARLY SUITED FOR THERMAL CONVERSION TO CARBON FILAMENTS [75] Inventors: Michael .1. Ram, West Orange;'John P. Riggs, Berkley Heights, both of [73] Assignee: Celanese Corporation, New York,

22 Filed: Apr. '17, 1972 [21] Appl. No.: 244,541

Related US. Application Data [63] Continuation-impart of Ser. No. 28,545, April 14,

1970, Pat. NO. 3,657,409. v 1

[52] US. Cl. 260/88.7 R, 260/855 S [51] Int. Cl. C08f 3/74, C081 15/22 [58] Field of Search 260/324, 88.7 R, 85.5 S;

OTHER PUBLICATIONS Studies on Acrylic Fiber-Characterization on Acrylic Filaments Spun With Various'Coa'gulation Media, Takeda, H. Kogyo Kagaku Zasshi, 67: 630632; 1964.

Primary Examiner-Jay H. W00

[57] ABSTRACT Improved acrylic filaments are provided having an internal structure capable of yielding improved carbon filaments upon thermal conversion. The acrylilc filaments of the present invention are formed by a specifically defined wet spinning process which produces filaments having a highly fibrillar and dense internal structure. Upon thermal stabilization and subsequent thermal treatment at 1,000 to 2,000C. a highly developed microporous structure is inherently imparted to the resulting carbon filaments wherein a large number of elongated micropores are disposed between a highly fibrillar internal structure. The resulting carbon filaments have an internal structure capable of increasing the amount of work required to break the same via crack diversion during fracture. Satisfactory strength properties are exhibited even if the internal structure of the improved carbon filaments is accompanied by the presence of gross inhomogenities and structural flaws, such as commonly encountered in carbon filaments of the prior art.

2 Claims, 2 Drawing Figures PATENTEnuuv 5l974 3.846.833.

sum 1w 2 PATENTED 5 I974 amaurz ACRYLIC FILAMENTS WHICH ARE PARTICULARLY SUITED FOR THERMAL CONVERSION TO CARBON FILAMENTS CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION Acrylic filaments have been formed via a variety of dryspinning and wet spinning processes in the prior art. Such acrylic filaments have been utilized,inter alia, as fibrous precursors which upon thermal treatment are capable of yielding carbon filaments. It has generally been recognized that the structure of the carbon filaments is influenced to some degree by the nature of the fibrous material which is thermally converted into the carbon filaments and by the processing conditions utilized during the thermalconversion. It has also been recognized that carbon filaments are known to possess an internal structure which is somewhat fibrillar innature and that some micropores (i.e., microvoid s) in addition to the usual structural flaws may be detected within the same; See, for instance, the article by R. Perret and W. Ruland appearing in J. Appl. Cryst., Vol. 3, Pages 525-532 (1970), entitled The Microstructure of PAN-Base Carbon Fibres.

In the search for high performance materials considerable interest has been focused upon carbon fibers. Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as a high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high modulus and high tensile strength. Carbon fiber reinforced composites are commonly formed by incorporating carbon filaments in a resinous or metallic matrix. Representative uses for carbon fiber reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels and ablative materials for heat shields or re-entry vehicles,

etc.

Heretofore, those material scientists interested in attempting to improve the internal structure of carbon filaments have directed their attention largely to elimination of strength flaws within the same. See, for instance, the article by J. W. Johnson and D. J. Thorne appearing in Carbon, Vol. 7, Pages 659-661 (1969), entitled Effect of Internal Polymer Flaws on Strength of Carbon Fibres Prepared From an Acrylic Precursor, and the article by John W. Johnson appearing in Applied Polymer Symposia, Vol. 9, Pages 229-243 (1969), entitled Factors Affecting the Tensile Strength of Carbon-Graphite Fibres) The present invention provides a novel route to the improvement of carbon filaments. The acrylic filaments of the present invention possess an internal structure unlike that exhibited by acrylic filaments of the prior art and are capable upon thermal treatment of yielding improved carbon filaments as discussed in detail hereafter.

It is an object of the invention to provide improved acrylic filaments.

It is an object of the invention to provide acrylic filaments possessing an improved internal structure.

' It is an object of the invention to provide acrylic filaments which upon thermal conversion yield carbon filaments capable of substantial crack diversion upon fracture.

It is an object of the invention to provide an improved highly oriented acrylic filament possessing an unusually dense and highly developed fibrillar internal structure.

It is another object of the invention to provide an improved acrylic filament possessing an internal structure which upon thermal conversion to a carbon filament facilitates exhibition of highly satisfactory strength properties even if accompanied by the presence of structural flaws such as commonly encountered in carbon filaments of the prior art.

These and other objects as well as the scope, nature, and utilization of the invention will be apparent from the following description and appended claims.

SUMMARY OF THE INVENTION An improved acrylic filament which is particularly suited for thermal conversion to a carbon filament formed in accordance with the process comprising:

a. providing a spinning solution having a low shear viscosity of about to 3,000 poise measured at 25C. comprising (1) a fiber-forming acrylic polymer in a concentration of about 15 to 30 percent by weight based upon the total weight of the solution selected from the group consisting of an acrylonitrile homopolymer and an acrylonitrile copolymer which contains at least about mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized'therewith, and (2) a dimethylacetamide solvent,

b. extruding the spinning solution into an essentially nonaqueous coagulation bath having a temperature of about 0 to 45C. consisting essentially of about 55 to 85 percent by weight of ethylene glycol and about 15 to 45 percent by weight of dimethylacetamide,

' c. washing and resulting as-spun filament in water to remove dimethylacetamide from the same, the washing being initially conducted for at least 25 seconds with the water at a temperature of about 10 to 50C., and d. drawing the washedfilament to increase the orientation thereof.

DESCRIPTION OF DRAWINGS FIG. 1 is a photograph at a magnification of 2,500X which illustrates the substantially round cross-sectional configuration of acrylonitrile homopolymer filaments of the present invention which were formed in accordance withthe procedure described in the Example.

FIG. 2 is a schematic view of a representative apparatus arrangement suitable for forming the acrylic filaments of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The improved acrylic filaments of the present invention may be formed in accordance with embodiments of the process described in our commonly assigned contain at least about 95 mol percent of recurring acrylonitrile units and up to about mol percent of one or more monovinyl units copolymerized therewith. Representative monovinyl units which may be incorporated in the acrylonitrile copolymers include styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloridine, vinylidene chloride, vinyl pyridine, and the like. The acrylic polymers may be formed by standard polymerization processes which are well known in the art. Minor quantities of preoxidation or graphitization catalysts may optionally be incorporated in the bulkacrylic polymer prior to spinning.

The solvent utilized to form the spinning solution may be dimethylacetamide. The solvent is sometimes identified as N,N-dimethylacetamide or DMAC, and has the chemical formula CH CON(CI-I The standard technical or commercial grade of dimethylacetamide may be employed as the solvent in the formation of the spinning solution.

The spinning solution may be prepared by dissolving sufficient acrylic polymer in the dimethylacetamide solvent to yield a solution suitable for extrusion containing from about 15 to 30 percent acrylic polymer by weight based upon the total weight of the solution, and preferably from about 18 to 25 percent by weight. In a particularly preferred embodiment of the invention the spinning solution contains the acrylic polymer in a concentration of about 20 to 22 percent by weight based upon the total weight of the solution. The low shear viscosity of the spinning solution should be within the range of about 80 to 3,000 poise measured at 25C. and preferably within the range of about 125 to 1,500 poise measured at 25C. If thespinning solution low shear viscosity is much below about 80 poise measured at 25C., spinning breakdowns commonly occur. If the spinning solution low shear viscosity is much above about 3,000 poise measured at 25C., extremely high spinning pressures are required and plugging of the extrusion orifice may occur.

In a preferred acrylic filament formation technique .the spinning solution additionally contains about 0.1 to

5.0 percent by weight based upon the total weight of the solution, and preferably about 0.5 to 2 percent by 'weight based upon the total weight of the solution of 250 hours. A solution containing an even lesser concentration of acrylonitrile homopolymer and no lithium chloride (i.e., 20 parts by weight polymer, and parts by weight dimethylacetamide) tends to increase in viscosity upon standing and exhibits a low shear viscosity of about 1,000 poise measured at 25C. after about 2 /2 hours. The lithium chloride may be dissolved in the dimethylacetamide solvent either simultaneously with the acrylic polymer or before or after the acrylic polymer is dissolved therein. Minor quantities of preoxidation or graphitization catalysts may optionally bekincorporated in the spinning solution.

The spinning solution is preferably filtered, such as by passage through a plate and frame press provided with an appropriate filtration medium, prior to wet spinning in order to assure the removal of any extraneous solid matter which could possibly obstruct the extrusion orifice during the spinning operation.

The spinning solution containing the fiber forming acrylic polymer dissolved therein is extruded into a coagulation bath under conditions capable of forming the acrylic filaments of the present invention having a highly dense and highly fibrillar internal structure which is capable upon subsequent thermal treatment of yielding improved carbon filaments.

It has been found that acrylic filaments of the present invention are produced when an essentially nonaqueous coagulation bath is utilized having a temperature of about 0 to 45C. (preferably about 10 to 35C.) which consists essentially of about 55 to percent by weight of ethylene glycol and about 15 to 45 percent by weight of dimethylacetamide. When employing dimethylacetamide in the coagulation bath in concentrations much greater than about 45 percent by weight, then filament breakage tends to occur at the spinneret. When employing dimethylacetamide in the coagulation bath in concentrations much less than about 15 percent by weight, then the resulting filaments tend to lose their substantially round crosssection and have a tendency to exhibit a more pronounced bean-shaped configuration. In a preferred embodiment of the invention the coagulation bath consists essentially of about 60 to 75 percent by weight of ethylene glycol and about 25 to 40 percent by weight of dimethylacetamide. In a particularly preferred embodiment of the invention the coagulation bath consists essentially of about 60 percent by weight of ethylene glycol and about 40 percent of dimethylacetamide. At the relatively low coagulation bath temperatures employed the coagulation rate tends to be relatively slow and to enhance the formation of the desired fiber internal structure.

The temperature of the spinning solution at the time of its extrusion should be within the range of about 10C. to about C., and preferably at about 20 to 30C. In a particular preferred embodiment of the invention the spinning solution is provided at room temperature, e.g., about 25C., which desired facilitates expeditious handling and storage of the same.

The spinneret utilized during the extrusion may contain a single hole through which a single filament is extruded, and preferably contains a plurality of holes whereby a plurality of filaments may be simultaneously extruded in yarn or tow form. For instance, tows of up to 20,000, or more, continuous filaments may be formed. The spinneret preferably contains holes having a diameter between about 50 to microns when producing relatively low denier filaments having an asspun denier of about 8 to 24 denier per filament, and holes of about 300 to 500 microns when producing relatively high denier filaments having an as-spun denier of about 100 to l,500 denier per filament. Extrusion pressures between about 100 and700 psig may be conveniently selected, and preferably between about 100 and 400 psig. Spinning or extrusion speeds of about 0.5 to 10 meters per minute (e.g., 3 to 6 meters per minute) may be employed.

Throughout the extrusion process the coagulation bath is preferably circulated. A relatively constant composition within the coagulation bath may be maintained throughthe continuous withdrawal and purification of the same. Alternatively, additional ethylene glycol may be continuously added to the coagulation bath to preserve the desired proportion of dimethylac etamide to ethylene glycol within the same. The length of the coagulation bath'is adjusted so that the resulting as-spun filaments are present withinthe coagulation bath for a residence time of at least about 6 seconds. For instance, residence times of about 6 to 300 seconds may be conveniently selected. Residence times less than about 6 seconds tend to result in an insufficiently developed dense fibrillar structure within the as-spun filaments. Residence times for the as-spun filaments in the coagulation bath in excess of 300 seconds tend to yield no commensurate advantage. Particularly preferred residence times for the as-spun filament in the coagulation bath range from about 6 to 50 seconds.

The resulting as-spun filaments is next washed with water to remove dimethylacetamide solvent from the At temperatures below about 10C. the washing procedure tends to be unduly slow. The residual dimethylacetamide contentof the washed acrylic filaments preferably is no more than about 5 percent dimethylacetamide by weight, and most preferably no more than 1 about 0.1 percent by weight, prior to subsequent prosame. The as-spun filament is preferably washed with water until substantially all residual amounts of solvent, coagulation bath, and inorganic compound (e.g., lithium chloride), if any, are'remov'ed from the same. It is essential that the filament first be exposed to a rela tively cool water wash medium at a temperature of about 10 to 50C. and preferably at about 10 to 30C., and most preferably at room temperature (e.g., about 25C.), for at least about 25 seconds. The entire wash treatment may be conducted at a temperature within the range of about 10 to 50C. Alternatively the washing of the filament may be subsequently continued at a more highly elevated, temperature, e.g., in excess of about 50C. to remove additional solvent. In a preferred embodiment of the invention the initial cold water wash is conducted for at least about 50 seconds.

When the entire wash is-conducted at a relatively cool wash temperature of about 10 to 50C., wash times of about 25 to 240 seconds and preferably about 50 to 120 seconds are commonly utilized depending upon the filament denier. Longer wash times tend to yield no commensurate advantage.

- It has been found that the initial cool water wash described above is essential in order to preserve the requisite fiber internal structure in the acrylic filament. During the cool water wash a one-way transfer of residual quantities of the dim'ethylacetamide spinning solvent out of the filament is believed to be promoted to the substantial exclusion of the passage of the molecules of the water wash medium into the filament. It has been foundthat if the as-spun filament is initially washed at a temperature substantially higher than about 50C., then the resulting washed filament tends to contain a significant number of macrovoids and tends to flatten.

cessing.

The water wash treatment is conveniently conducted in an in-line operation with the filament after it leaves the coagulation bath being continuously I passed through a water wash medium which is continuously regenerated. Conventional filament wash rolls may be utilized. The filament alternatively may be washed with water while wound upon a perforated bobbin, or by the use of other washing means as will be apparent to those skilled in the art;

The as-spun and washed acrylic filament is drawn or stretched from about 1.5 times its original length up to the point at which the filament breaks to orient the same and to thereby enhance its tensile properties.

.Total draw ratios above about 1.5:1 to 15:1 may commonly be selected. The drawing is commonly conducted at an elevated temperature and preferably at a total draw ratio of between about 3:1 and 12:1. The dense and fibrillar internal filament structure makes possible the use of the relatively high total draw ratios indicated. As will be apparent to those skilled in the'art, the drawingof the as-spun and washed acrylic filament may be conducted by a variety of techniques. For instance, it is possible for the drawing to be conducted while the filament is (a) immersed in a heated liquid draw medium, (b) suspended in'a heated gaseous atmosphere, (e.g., at a temperature of about to 200C), or (c) in contact with a heated solid surface (e.g., at a temperature of about to C.) 1f desired, the total draw imparted to the filament may be conducted by a combination of the foregoing techniques When draw techniques (b) and (c) are utilized, it is essential that the acrylic filament be provided to the draw zone in an essentially dry form in order to avoid void formation. When draw technique (a) is employed, the acrylic filament is subsequently washed to remove the draw medium and is dried. Additionally, the liquid draw medium may also serve a washing and- /or coagulating function wherein residual quantities of dimethylacetamide are removed from the water washed fiber.

In a preferred embodiment of the invention the washed acrylic filament is at least partially drawn while immersed in a hot glycerin bath. In a particularly preferred embodiment of the invention the filament is drawn while immersed in a hot glycerin bath at a temperature of about 80 .to l 10C. and at a draw ratio of about 1.5:] to 3:1 (preferably at a temperature of in the art prior to a thermal conversion into improved carbon filaments.

The resulting acrylic filaments of the present invention may be converted to a stabilized or heat-resistant .and results in the formation of a cyclized and preoxidized product which exhibits a thermal stability not exhibited by the unmodified acrylic filaments. While it is possible that the stabilization reaction be conducted on a batch basis, it is preferable that the stabilization reaction be conducted on a continuous basis. Catalyzed stabilization reactions optionally may be selected. The exact stabilization temperatures employed will vary with the chemical composition of the acrylic filaments.

Preferred stabilization procedures are described in commonly assigned U.S. Pat. application Ser. No.

749,957, filed Aug. 5, 1968, of Dagobert E. Stuetz (now abandoned), and in U.S. Pat. No. 3,539,295, of Michael J. Ram, which are herein incorporated by reference. Other stabilization procedures capable of imparting thermal stability to the acrylic filaments may be selected. The highly fibrillar internal structure required to make possible the formation of improved carbon filaments is retained throughout the stabilization reaction.

The stabilized acrylic filaments maybe converted to improved carbon filaments by thermal treatment at a more highly elevated temperature of at least 1,000C., e.g., l,000 to 2,000C. in a non-oxidizing atmosphere. Preferably inert atmospheres such as nitrogen, argon and helium are employed. The stabilized acrylic filaments are "subjected to such highly elevated thermal treatment until carbon filaments containing at least 90 percent carbon by weight are formed, and preferably until carbon filaments containing at least about 95 percent carbon by weight are formed. In a more particularly preferred embodiment carbon filaments containing at least 98 percent carbon are formed. Carbon filaments of optimum tensile strength are formed when the maximum temperature provided in the heating zone is about 1,500 to l,900C. (e.g., 1,800C.). The carbon fibers are preferably formed on a continuous basis by continuous passage through a heating zone containing a non-oxidizing atmosphere and a temperature gradient in which the stabilized acrylic filaments are gradually raised to the maximum carbonization temperature. During the thermal treatment at l,000 to 2,000C. a highly developed microporous structure is inherently imparted to the resulting carbon filaments wherein a large number of elongated micropores of up to about 25 Angstroms (e.g., about to Angstroms) in thickness are disposed between the highly fibrillar internal structure which are largely preserved during the thermal treatment. The increased presence of the micropores within the highly fibrillar internal structure is confirmed by small angle X-ray analysis.

The carbon filaments which may be derived from the improved acrylic filaments of the present invention commonly exhibit a mean single filament Youngs modulus of about to 50 million psi. As discussed hereafter, the internal structure of the resulting carbon filaments facilitates exhibition of improved filament tenacity in spite of structural flaws such as those com- 8 monly encountered in the prior art. For instance, carbon filaments commonly possess flaws of 0.5 micron and larger as discussed in the Johnson, and Johnson and Thorne articles cited earlier.

The resulting carbon filaments have an unusually highly developed microporous and fibrillar internal structure which is capable of diverting a propagating crack during fracture thereby increasing the amount of work required to break the filament as evidenced by a mean apparent fracture surface energy (i.e., w) of at least 50 joules per square meter (e.g., 50 to 150, or more, joules per square meter). In a preferred embodiment the carbon filaments exhibit a mean apparent fracture surface energy of at least 60 joules per square meter. In a particularly preferred embodiment the carbon filaments exhibit a mean apparent fracture surface energy of at least 70 joules per square meter.

It has been found that carbon/ graphite filaments fail in a brittle fashion and that the Griffith failure criterion can be utilized to elucidate the fracture phenomenology. One form of the simple Griffith equation which has been applied to the fracture of many brittle materials is as follows:'

to the energy required to break primary chemical bonds. It is recognized, however, that in all but extremely rare cases of apparently brittle fracture, large amounts of plastic work are done at the crack tip, leading to observed values of ya much higher than would be expected from bond-breaking alone. See, for example, Fracture edited by H. Liebowitz, Academic Press, New York (1968). The apparent fracture surface energy, ya, is an intrinsic property of the carbon fiber and accordingly is dependent upon the internal physical structure of the carbon fiber.

A technique for the determination of the mean apparent fracture surface energy, ya, for a given carbon filament is described in detail below. Generally stated the WTB and C for a given carbon filament are determined and the mean apparent fracture surface energy is calculated therefrom.

1. Single carbon filaments individually are broken while immersed in glycerin and the stress, strain, and Youngs modulus determined by conventional fiber testing techniques. The use of .a glycerin bath minimizes the formationof secondary fracture surfaces which tend to be formed in an open atmosphere.

2. The resulting pair of ends for each broken filament is examined in the field of a scanning electron microscope and compared to assure a match thus insuring that a primary fracture surface is being examined.

3. The critical flaw, C, which initiated the fracture is located and the longest dimension on the primary fracture surface is measured.

The above procedure is repeated a number of times for a given sample of carbon filaments and the values obtained for WTB and C are plotted as log WTB vs. log

C. At least squares line'is drawn through the points assuming a slope of 1 in accordance with the theoretical prediction of the Griffith equation. The intercept of this line is accordingly log ('ya/rr) from which the mean ya, i.e., mean apparent fracture surface energy, is determined. It is recommended that at least four carbon filaments be broken as described above when determining the mean apparent fracture surface energy for .a given sample of carbon filaments. The results obtained are, of course, more statistically accurateas the number of breaks increases.

The mean apparent fracture surface energies of a wide variety of carbon filaments have been determined. In all instances, the mean apparent fracture surface energies were substantially below that of the improved carbon filaments which may be derived from the improved acrylic filaments of the present invention. The following Table I sets forth the average flaw size and mean apparent fracture surface energy obtained for representative carbon filaments.

that it is not essential that the improved acrylic fila" ments of the present invention be formed through the utilization of the exact processing parameters set forth in the Example.

EXAMPLE Twenty-two parts by weight of polyacrylonitrile homopolymer, 2 parts by weight of lithium chloride, and 76 parts by weight of industrial grade dimethylacetamide are slurried at room temperature for 120 min- TABLE I Carbon Carbon Single Average Apparent Fiber Source Fiber Designation FilamentYoungs Flaw Fracture Surface odulus Size Energy v Mpsi micron joules/sq.m.

Great Lakes Carbon 3T 31 0.8 33 Corp. 4T 39 0.8 31 ST 48 0.9 37 6T 60 0.9 26

Hercules, lnc. HT-S 39 0.7 4l AS 36 l.[ 42 HM-S 64 L4 Johnson & Thorne Acrilan 20 0.7 39

Carbon Fibers" Celanese A 1.] 33 B 35 0.5 29 C 35 1.4 29 D 37 1.4 l4

Derived from a dry spun Orlon acrylic precursor.

Derivccl from a wet spun Courtelle acrylic precursor (sodium thiocyanate spinning solution). Shown in FIG. 5, following Page 661. of Vol. 7, of Carbon. article by J. W. Johnson and D. .1. Thorne cited-earlier (wet spun from DMAC spinning solution employing an aqueous coagulation bath). Derived from various dry spun acrylic precursors.

An unusually high mean apparent fracture surface energy enables a larger average flaw size to be tolerated while still obtaining a high strength carbon filament. As

fracture is initiated and a propagating crack meets a micropore between fibrils, the crack is diverted and additional energy is consumed. For example, see the following Table II.-

Carbon filaments exhibiting a mean apparent fracture surface energy of at least joules/sq.m. and

which may be derived from the acrylic filaments of the present invention are claimed in our commonly assigned U.S. application Ser. No. 244,544, filed concurutes by use of a stirred vessel. The slurry is heated to a temperature of 100C. over a period of about minutes where it is mixed with agitation for 2 hours. The

The spinning solution is provid ed-in dope bomb 1- under an atmosphere of nitrogen at 20 psig. The spinning solution is conveyed to spinneret 2 via the line 4 where it is extruded into coagulation bath 6. The spinneret 2 is of the standard cup type and comprises a single circle of 400 holes each having a diameter of microns. V

The coagulation bath consists of 60 parts by weight ethylene glycol and 40 parts by weight dimethylacetamide and is provided at a temperature of 36C. The coagulation bath is caused to flow concurrently with coagulated filament 8 and is maintained at a relatively constant composition by the continuous addition of ethylene glycol to the same and the continuous withdrawal of a portion of the bath. The coagulation bath has a length of 37 inches and the coagulated filaments are maintained in the same for a residence time of about 9 seconds.

The coagulated filaments pass under guide 10 which is immersed in coagulation bath 6 and are conveyed to a skewed roll 1-2 and wash roll 14 which is partiallyimmersed in water bath 16 is maintained at 23C. The coagulated filaments are taken up on roll 12 at a rate of 6 meters per minute. The filaments are wrapped about skewed roll 12 and wash roll 14 for a residence time of about 125 seconds during which time the filaments are immersed in water for approximately 25 seconds and withdrawn with water adhering to the same during which time substantially all residual amounts of dimethylacetamide are removed from the same.

The washed filaments are next continuously passed through stretch bath 17 having a length of inches whichis provided with glycerin at 80C. Rollers 18 and 20 situated outside the stretch bath and rollers 22 and 24 immersed within the stretch bath guide the filaments substantially removed from the same. The water present in wash bath is circulated and is constantly regenerated. The washed filaments are next passed to skewed roll 32 and drying roll 34 where residual quantities of moisture areexpelled from the same. Drying roll 34 is steam heated and maintained at a constant temperature of approximately 95C.

The washed and dried filaments are next passed over a 2 foot heated draw shoe which is provided at a constant temperature of 145C. The residence time of the filaments while in contact with the hot shoe 36 is 1.25 second. The drawn filaments are collected on takeup roll 38 at a rate of 60 meters per minute.

The resulting drawn acrylic filaments possessing a highly oriented and fibrillar internal structure exhibit a denier per filament of 1.57, an average single filament tensile strength of 5.96 grams per denier, an average single filament initial modulus of 137 grams per denier, and an elongation of 8.64 percent.

FIG. 1 is a photograph at a magnification of 2,500 which illustrates the substantially round cross-sectional configuration of the resulting drawn acrylonitrile homopolymer filaments.

Two of the drawn filament bundles next may be plied to form a continuous length of acrylic fibrous material consisting of 800 continuous filaments. This resulting continuous length next may be subjected to a brief thermal pretreatment in accordance with the teachings of commonly assigned U.S.' application Ser. No. 17,962, filed Mar. 9, 1970 (now abandoned). More specifically, the continuous length is passed continuously through an oven provided with an air atmosphere at 195C. for a residence time of about 240 seconds 12 while maintaining the longitudinal tension thereon so that 10.5 percent shrinkage in length takes place.

The continuous length of thermally pretreated acrylic fibrous material next may be passed for about 180 minutes through a multiple roll oven provided with an air atmosphere at 266C. While passing through this oven the acrylic fibrous material is thermally stabilized and is rendered black and non-burning when subjected to an ordinary match flame. The resulting stabilized fibrous material retains its original fibrous configuration essentially intact, and contains a bound oxygen content of about 10.1 percent by weight when subjected to the Unterzaucher analysis.

The continuous length of stabilized filaments next may be converted to improved carbon filaments by passage through an Inductotherm induction furnace utilizing a 20 KW power source. The induction furnace comprises a water cooled copper coil and a hollow graphite tube suspended within the coil having a length of 38 inches and an inner diameter of 0.75 inch through which the continuous length of stabilized filaments is continuously passed. The copper coil which encompasses a portion of the hollow graphite tube is positioned at a location essentially equidistant from the respective ends of the graphite tube. An inert atmosphere of nitrogen is maintained within the induction furnace. Air is substantially excluded from the induction furnace by purging with nitrogen. The continuous length of stabilized filaments is passed through the induction furnace at a rate of about 3 inches per minute. A longitudinal tension of 0.2 grams per denier is exerted upon the continuous length of fibrous material as it passes through the induction furnace. The fibrous material is at a temperature of about 150C. as it enters the induction furnace and is raised to a temperature of 800C. in about 150 seconds, and from 800 to 1,500C. in about 200 seconds where it was maintained at 1,500 i 25C. for about 48 seconds.

The resulting carbon filaments contain in excess of 98 percent carbon by weight, and are found to possess an average flaw size of 2.7 microns. The mean apparent .specific gravity per filament of about 1.75, a denier per filament of 0.78, a mean single filament tensile strength of 350,000 psi, a mean single filament Youngs modulus of 44 million psi, and an elongation of 0.78 percent.

Although the formation of the improved acrylic filaments of the present invention has been described with preferred embodiments, it is to be understood that variations and modifications may be employed in the acrylic filament formation technique without departing from the concept of the present invention.

We claim:

1. An improved acrylicfilament which is particularly suited for thermal conversion to a carbon filament formed in accordance with the process comprising:

a. providing a spinning solution having a low shear viscosity of about 125 to 1,500 poise measured at 25C. comprising (1) a fiber-forming acrylic polymer in a concentration of about 18 to 25 percent by weight based upon the total weight of the solution selected from the group consisting of an acrylonitrile homopolymer and an acrylonitrile copolymer which contains at least about mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith, (2) lithium chloride in a concentration of about 0.1 to 5 percent by weight based upon the total weight of the solution which lowers and preserves upon standing the viscosity of the solution, and (3) a dimethylacetamide solvent,

b. extruding said spinning solution into an essentially non-aqueous coagulation bath having a temperature of about to 35C. consisting essentially of about 55 to 85 percent by weight of ethylene glycol and about to 45 percent by weight of dimethylacetamide to form an as-spun filament which is present in said coagulation bath for a residence 0 suited for thermal conversion to a carbon filament formed in accordance with the process described in claim 1 wherein said fiber-forming acrylic polymer is an acrylonitrile homopolymer. 

1. AN IMPROVED ACRYLIC FILAMENT WHICH IS PARTICULARLY SUITED FOR THERMAL CONVERSION TO A CARBON FILAMENT FORMED IN ACCORDANCE WITH THE PROCESS COMPRISING: A. PROVIDING A SPINNING SOLUTION HAVING A LOW SHEAR VISCOSITY OF ABOUT 125 TO 1,500 POISE MEASURED AT 25*C. COMPRISING (1) A FIBER-FORMING ACRYLIC POLYMER IN A CONCENTRATION OF ABOUT 18 TO 25 PERCENT BY WEIGHT BASED UPON THE TOTAL WEIGHT OF THE SOLUTION SELECTED FROM THE GROUP CONSISTING OF AN ACRYLONITRILE HOMOPOLYMER AND AN ACRYLONITRILE COPOLYMER WHICH CONTAINS AT LEAST ABOUT 85 MOL PERCENT OF ACRYLONITRILE UNITS AND UP TO ABOUT 15 MOL PERCENT OF ONE OR MORE MONOVINYL UNITS COPOLYMERIZED THEREWITH, (2) LITHIUM CHLORIDE IN A CONCENTRATION OF ABOUT 0.1 TO 5 PERCENT BY WEIGHT BASED UPON THE TOTAL WEIGHT OF THE SOLUTION WHICH LOWERS AND PRESERVES UPON STANDING THE VISCOSITY OF THE SOLUTION, AND (3) A DIMETHYLACETAMIDE SOLVENT, B. EXTRUDING SAID SPINNING SOLUTION INTO AN ESSENTIALLY NONAQUEOUS COAGULATION BATH HAVING A TEMPERATURE OF ABOUT 10* TO 35*C. CONSISTING ESSENTIALLY OF ABOUT 55 TO 85 PERCENT BY WEIGHT OF ETHYLENE GLYCOL AND ABOUT 15 TO 45 PERCENT BY WEIGHT OF DIMETHYLACETAMIDE TO FORM AN ASSPUN FILAMENT WHICH IS PRESENT IN SAID COAGULATION BATH FOR A RESIDENCE TIME OF ABOUT 6 TO 300 SECONDS, C. WASHING THE RESULTING AS-SPUN FILAMENT IN WATER TO REMOVE DIMETHYLACETAMIDE AND LITHIUM CHLORIDE FROM THE SAME, THE SAID WASHING BEING INITIALLY CONDUCTED FOR AT LEAST 25 SECONDS WITH SAID WATER AT A TEMPERATURE OF ABOUT 10* TO 50*C., AND D. DRAWING SAID WASHED FILAMENT TO INCREASE THE ORIENTATION THEREOF.
 2. An improved acrylic filament which is particularly suited for thermal conversion to a carbon filament formed in accordance with the process described in claim 1 wherein said fiber-forming acrylic polymer is an acrylonitrile homopolymer. 